In the prior art, a vehicle security system is known in which intrusion into a vehicle is detected with the use of an ultrasonic wave generated from a frontmost part of an approximately center of the vehicle room, and alarming is made (for example, see Patent Document 1).
Further, a vehicle intrusion detection system is known in which a reflected wave obtained from reflection of a transmission wave, which is transmitted in a vehicle, from a body in the vehicle is analyzed, and intrusion into the vehicle is detected (for example, see Patent Document 2).
Patent Document 1: Japanese Laid-Open Patent Application 2002-53011
Patent Document 2: Japanese Laid-Open Patent Application 2003-167050